About Supply & Return

Supply & Return is a free set of heating, ventilation and air conditioning calculators for homeowners, HVAC apprentices and technicians, and contractors who want to check a number before they quote, buy or install. It covers the questions that come up on every job: how big a unit a room needs, how much heat a house loses, how much air a system must move, what size duct carries it, whether a more efficient unit pays for itself and how much fresh air a tight house needs.

The Calculators

  • Room BTU Calculator — room air conditioner sizing from the ENERGY STAR table with sun, occupant, kitchen and ceiling adjustments.
  • Heat Loss Calculator — design heating load from walls, windows, ceiling, R-values and air leakage.
  • Airflow CFM Calculator — airflow from sensible load and temperature difference, air changes per hour or tonnage.
  • Duct Size Calculator — round duct size by friction rate or velocity, standard sizes and rectangular equivalents.
  • SEER2 Savings Calculator — yearly electricity use and cost by efficiency rating, with simple payback.
  • Ventilation Rate Calculator — whole-house mechanical ventilation under ASHRAE 62.2.

Why We Built It

The most expensive HVAC mistakes are sizing mistakes: an air conditioner chosen two sizes too large, a furnace replaced like-for-like in a house that has since been insulated, a branch duct too small for the room at the end of it. Many of them come from rules of thumb applied without arithmetic. Our calculators make the arithmetic quick and show the steps, so you can see which assumption drives the answer.

Methods and Sources

Room sizing follows the ENERGY STAR room air conditioner guidance. Heat loss uses conduction through each surface (area × temperature difference ÷ R-value) and air leakage at 0.018 BTU per cubic foot per degree Fahrenheit. Airflow uses the standard-air sensible heat factor of 1.08. Duct sizing uses the ASHRAE friction equation for smooth round galvanised duct and the Huebscher equivalent-diameter equation. Efficiency savings use the SEER2 definition, and ventilation rates use ASHRAE Standard 62.2-2016, with the 2010 rate shown for comparison. Each page states its assumptions and its limits.

Who Uses It

Homeowners comparing quotes, apprentices learning load and airflow calculations, service technicians checking a complaint on site, energy auditors explaining results to clients and contractors sanity-checking a design before submitting it.

Keeping It Accurate

We test every calculator against worked examples, such as 400 CFM at 0.08 in. w.c. per 100 ft needing about a 10 in. round duct, and against published tables. When standards change, as with the move from SEER to SEER2 in 2023, we update the tools and the explanations. If a result disagrees with a manufacturer's table or your own calculation, tell us.

Not a Substitute for a Design

These tools are for estimates and education. Equipment sizing for permits and installations should use a full calculation such as ACCA Manual J, S and D, by a qualified professional, and gas, electrical and refrigerant work must be done by licensed technicians.

Independence and Funding

Supply & Return is not affiliated with any equipment manufacturer, distributor or utility. It is free and supported by advertising; advertisers have no influence over the calculators or guides.

Contact

Corrections and suggestions: logan.luo@adsnova.cn. Your inputs stay in your browser; see the Privacy Policy.